1. Technical Field
This invention relates to electret materials, and more particularly, to an electret speaker and a method of manufacturing the same.
2. Background
An electrostatic speaker operates on the principle of Coulomb's law that two conductors with equal and opposite charge may generate a push-pull force between them. The push-pull electrostatic force may cause vibration of a diaphragm with static charge, and thereby generating sound. An electrostatic speaker may typically include two porous conductive plates or electrode plates and a diaphragm placed between and driven by the conductive plates or electrode plates. The electrodes and the diaphragm are separated by air gaps to provide space for the diaphragm to vibrate. The diaphragm is usually thin and light, and thus making the electrostatic speaker superior to other types of speakers, such as dynamic, moving-coil or piezoelectric speakers, with respect to its transition response, expansion capability in high frequency, smoothness of sound, acoustic fidelity and low distortion. Moreover, the diaphragm must be statically electrical charged, allowing induced static force to drive the diaphragm under the electric field formed by electrode plates when signals are supplied to the electrode plates.
With the simple structure, electrostatic speakers may be manufactured in various sizes to accommodate increasing demand for small and thin electronic devices. However, a conventional electrostatic speaker adopts DC-DC converter to provide the static charge on the diaphragm made by conductor. Considering the size, cost and power consumption of DC-DC converters, electret materials have been developed to replace DC-DC converters. Electret (formed of elektr- from “electricity” and -et from “magnet”) is a dielectric material that has a quasi-permanent electric charge or dipole polarization. An electret generates internal and external electric fields, and can be the electrostatic equivalent of a permanent magnet. See, e.g., G. M. Sessler, Topics in Applied Physics, vol. 33, Chapter 1, pg. 1 (1980), and U.S. Pat. No. 4,288,584 (Mishra). The electret charge may include real charge (such as surface and/or space charges) and/or polarization of dipoles. Real charges comprise layers of trapped positive and negative charge carriers.
An exemplary electret speaker is illustrated in FIG. 1, which may include porous electrode plates 6a and 6b, and a electret diaphragm 4. The electrodes 6a and 6b may have a number of openings 61a and 61b on each electrode plate having a porosity of at least 30 percent. The electrode plates 6a and 6b may be made of metals or plastic materials coated with a conductive film. The openings 61a and 61b may be provided for allowing sound waves to pass through. The electret diaphragm 4 may include a conductive layer 2 sandwiched between electret layers 1a and 1b. The electret layers 1a and 1b may contain either positive charges or negative charges, or may be oriented to have opposite dipole polarization in the normal direction of electret diaphragm 4. The electrode plates 6a and 6b, and electret diaphragm 4 may be held in place by holding members 5a and 5b. The holding members 5a and 5b may be made of insulating materials. The electrode plates 6a and 6b are separated from the electret diaphragm by insulating elements 51a, 51b, 52a and 52b. In operation of an electret speaker, each signal source 7a and 7b outputs an alternating signal to the electrode plates 6a and 6b via conductive lines 8a and 8b respectively. The signals cause a time-varying electric field to develop between the electrode plates 6a and 6b and the electret layers 1a and 1b, thus resulting in a electrostatic force between electrode plates 6a and 6b and electret diaphragm 4. The electrostatic force may cause the electret diaphragm 4 to vibrate to generate sound. The resultant sound waves may pass through holes 61a and 61b, therefore, the sound waves could be heard outside the electret speaker.
However, for an electret speaker to enhance its acoustic fidelity and low distortion, it requires an electret material with excellent electret properties and also a delicate process to fabricate a thin electret-metal-electret structure. It has previously been known to make electrets from various polymers. Charges are generated in the polymer electrets film by subjecting it to a DC corona treatment. Of the various polymers that have been proposed the fluoropolymers, such as poly(tetrafluoroethylene) (PTFE), and fluorinated ethylene propylene (FEP), have been found to give rise to stable electret properties even at high temperatures and high humility. However, fluoropolymers may be expensive and may require certain processing techniques. Fluoropolymers may possess low friction coefficients on the surface, so that there is no seizure or stick-slip action. Therefore, these fluoropolymers do not adhere well to metals and are not suitable for being fabricated into a electret diaphragm in the electret speaker system. It has also been known previously to make electret from non-fluoropolymers, such as polystyrene (PS), polycarbonate (PC), polyvinyl chloride (PVC), polymethylmethacrylate (PMMA) and cyclic olefin copolymer (COC) These non-fluoropolymers mentioned above may be applied to fabricate as the electret diaphragms much easier than those fluoropolymers in electret speaker application. However, the initially high surface potential of electrets made from these non-fluoropolymers reduces relatively rapidly. This applies especially when the humidity is high. Therefore, there is still a need to make non-fluoropolymer electret whose electret properties are retained for longer period of time.
U.S. Pat. No. 4,654,546 and U.S. Pat. No. 6,852,402 described producing a cellular ferroelectret film made of polypropylene. During corona-charging process, the electric field causes dielectric breakdown inside each polymer cell: the dielectric breakdown deposits equal and opposite electrical charges on opposite interior surfaces of the cell. Each cell in the cellular ferroelectret which has space charges on its walls acts like a dipole. Therefore, the cellular ferroelectret acts like a dipolar electret with a large dipole moment and gets better electret properties than those solid electrets.
In the prior art, voids in cellular ferroelectret are formed by spontaneously open up in polymers that contain tiny foreign particles such as silicates (“sand”) when those polymers are highly stretched. Simultaneous or sequential stretching in two perpendicular directions results in films with lens-shaped voids. Those voids are often too flat for efficient charging by means of internal micro-plasma discharges, because the plasma electrons cannot be accelerated sufficiently to ionize the gas molecules. Additional, it may be difficult to control the thickness of a film when a film-stretching technique is used to make a cellular ferroelectret.